Ethylene-vinyl alcohol copolymers (hereinafter, may be abbreviated as “EVOH(s)”) are useful polymeric materials that are superior in barrier properties against various types of gases such as oxygen, oil resistance, antistatic properties, mechanical strength and the like; therefore, EVOHs have been molded into films, sheets, containers, packaging materials and the like, and widely used as various types of packaging materials and the like. In particular, laminates including an EVOH layer and other thermoplastic resin layer are known to be useful as packaging materials for boiling sterilization or retort sterilization of foods.
However, when the laminates are subjected to generally employed boiling or retorting processing using hot water, water penetrates into the EVOH layer during the processing, leading to deterioration of mechanical properties of the EVOH layer. As an improved method, blending the EVOH with a polyamide resin (hereinafter, may be abbreviated as “PA”) having superior hot water resistance has been conventionally employed (hereinafter, suitability for such boiling or retorting processes may be also referred to as “retort resistance”), and in these days, as a method for further improving the retort resistance, the following methods have been developed: a method involving laminating a layer formed from a resin composition having the mass ratio of EVOH/PA of 55/45 or more and 97/3 or less as an outermost layer, and a layer formed from a thermoplastic resin having low moisture permeability as an inner layer (see Japanese Unexamined Patent Application, Publication No. H10-80981); a method involving incorporating a metal compound and/or a boric acid compound into an intermediate layer formed from a composition containing EVOH and PA (see Japanese Unexamined Patent Application, Publication No. H4-131237); and a method involving forming an intermediate layer from a composition containing two types of EVOHs and PA (see Japanese Unexamined Patent Application, Publication No. H6-23924).
However, in the resin compositions containing EVOH and PA, a crosslinking reaction may proceed between a hydroxyl group or a terminal carboxyl group of the EVOH and an amide group, a terminal amino group or a terminal carboxyl group of the PA, leading to nonuniformity of a resin viscosity, and thereby resulting in the generation of burnt deposits within an extruder, a screw and a die to be remarkable during melt molding for a long time period, and the like.
Such burnt deposits generated within the extruder, the screw and the die may remain therein for a certain time period and thereafter contaminate molded articles during a continuous operation for a long time period. The burnt deposits thus contaminating the molded articles not only deteriorate an appearance, but also cause defects to be formed, resulting in deterioration of various mechanical properties. Typically, in order to prevent the burnt deposits from contaminating molded articles, it is necessary to periodically shut down the operation and to carry out disassembly for cleaning of a line of extrusion equipment. However, an increase of the frequency of this procedure leads to not only an increase in production costs, but also consumption of materials required for the shutdown and restarting as well as a loss of production time; therefore, improvements have been desired in light of both resources and costs.
Although the technologies disclosed in the aforementioned documents achieves an improvement of the retort resistance, the technologies are insufficient in terms of the inhibition of the generation of burnt deposits within a molding machine in the operation for a long time period.
In addition, a production method of EVOH has been known in which crotonaldehyde is coexisted in a polymerization step of ethylene and vinyl acetate (see Japanese Unexamined Patent Application, Publication No. 2007-31725). When crotonaldehyde is coexisted in the polymerization step as described above, the adhesion of scales in the interior of a polymerization reactor can be suppressed, and as a result, fish eyes and the like in EVOH films resulting from the scales detached and contaminated in the polymer can be reportedly reduced.
However, in the production method disclosed in the aforementioned document, crotonaldehyde added in the polymerization is consumed in the polymerization step and a subsequent saponification step, and further washed away with water in a washing step and the like; therefore, crotonaldehyde hardly remains in a finally obtained EVOH-containing resin composition. Therefore, the resultant EVOH-containing resin composition is believed to fail to achieve the effect of reducing the fish eyes and the like in EVOH films described above.
As described in the foregoing, both improvement of the retort resistance of the resin composition and preclusion of the unfavorable appearance due to burnt deposits formed in an operation for a long time period has been difficult based on the conventional technologies.